When data having the same content is intended to be transmitted to a plurality of wireless slave stations, multicast that is independent of the number of transmission destinations is more efficient than unicast in terms of resource utilization efficiency.
As one example, the basic protocol for multicast transmission is defined in “IEEE Standard 802.11-2012” that is IEEE 802.11 WLAN standard specification. However, the receipt from a plurality of devices fails to be confirmed in the protocol defined in this standard, and thus it may be impossible to improve the communication quality by retransmission control using the acknowledgement.
In the “IEEE802.11aa-2012” that is an amendment standard, the multicast extension with acknowledgement is defined, and the effect of improving the communication quality by retransmission control using the acknowledgement is introduced. However, in this extension method, the acknowledgement of multicast is performed individually for each group or member. Thus, with the increase in the number of terminals to be transmitted by multicast, the overhead due to the use of communication resources increases, resulting in the limit of the resource utilization efficiency that is an advantage of multicast.
A protection mechanism for multi-user MIMO communication that allows a reservation message to be transmitted to a plurality of devices simultaneously by multicast and allows the plurality of devices to transmit each confirmation message on a channel simultaneously in response to the reservation message thereby multiplexing the confirmation messages is developed (e.g., refer to Patent Literature 1). However, the spatial multiplexing in the protection mechanism is achieved on condition that the contents of the confirmation messages to be used are all the same. In other words, the protection mechanism as described above may be not applicable to a frame that is likely to have different contents for each terminal such as confirmation of receipt of multicast data.